The light-conducting process in an electrophotographic photoreceptor comprises:
(1) a step of generating an electric charge by exposure, and PA1 (2) a step of transporting the electric charge.
As an example of conducting both of the steps (1) and (2) in one substance, there is a selenium photosensitive plate. On the other hand, as an example of conducting the steps (1) and (2) separately in different substances, a combination comprising amorphous selenium and poly-N-vinylcarbaxole has been well known. The method of conducting the steps (1) and (2) separately in different substances is advantageous in that this may broaden the range of materials available for use in electrophotographic photoreceptors along with improving the electrophotographic characteristics, such as sensitivity and acceptable potential, of electrophotographic photoreceptors and, additionally, substances preferable for formation of coating films on photoreceptors may be selected from a broad range.
As already described by Carson in U.S. Pat. No. 2,297,691, a photoconductive material is used in electrophotography, having a substance which is insulating in the dark and whose electric resistance may vary in accordance with the imagewise exposure imparted thereto, coated on a support. In general, the photographic material is first processed to have a uniform surface charge in the dark, after dark adaptation for a while. Next, the material is imagewise exposed through a light-irradiating pattern which has the effect of reducing the surface charge in accordance with the relative energy contained in the various parts of the light-irradiating pattern. The surface charge or electrostatic latent image that remains on the surface of the photoconductive substance layer (electrophotographic photosensitive layer) is then converted into a visible image after the surface has been brought into contact with an appropriate charge-detecting and displaying substance, i.e., a toner.
The toner is contained in an insulating liquid or a dry carrier, and in either case, this may be applied to the surface of the electrophotographic photosensitive layer in accordance with the charge pattern formed. The displaying substance (toner) thus applied to the image surface may be fixed by conventional means such as heat, pressure or solvent vapor. The electrostatic latent image may be transferred to a second support (for example, paper or film). In the same manner, the electrostatic latent image transferred to the second support may be developed thereon. Electrophotography is one of image-forming methods where any desired images are formed in accordance with the processes mentioned above.
In such electrophotography, fundamental characteristics required for electrophotographic photoreceptors include, for example, the following: (1) they can be charged to a pertinent potential in the dark, (2) they barely lose the charge in the dark, and (3) they can rapidly release the charge after light-irradiation.
The above-mentioned inorganic substances which have heretofore been used in electrophotography have not only various merits but also various demerits, as a matter of fact. For example, selenium which is widely used at present may sufficiently satisfy the above-mentioned conditions (1) to (3), but the manufacturing is difficult and complicated and the manufacturing cost is high. In addition, the manufacturing process has no flexibility and the electrophotographic photoreceptor can barely be formed into a belt-shaped product. Further, as the electrophotographic photoreceptor is sensitive to heat and mechanical shock, special care is required for handling the same. Thus selenium has various defects. Cadmium sulfide and zinc oxide are used as an electrophotographic photoreceptor, after being dispersed in a resin as a binder. However, these electrophotographic photoreceptors have mechanical defects in smoothness, hardness, tensile strength and abrasion resistance, and so these cannot be repeatedly used as they are.
Recently, electrophotographic photoreceptors having various organic substances have been suggested in order to overcome the defects of the inorganic substances, and some have been put to practical use. For example, there are an electrophotographic photoreceptor comprising poly-N-vinylcarbazole and 2,4,7-trinitrofluoren-9-one (U.S. Pat. No. 3,484,237); an electrophotographic photoreceptor having poly-N-vinylcarbazole sensitized with a pyrylium salt dye (JP-B-48-25658) (the term "JP-B" as used hereinmeans an "examined Japanese patent publication"); and an electrophotographic photoreceptor having as a main component an eutectic complex comprising a dye and a resin (JP-A-47-10735) (the term"JP-A" as used herein means an "unexamined published Japanese patent application"). In addition, electrophotographic photoreceptors having an azo pigment as a main component have widely been studies recently and various proposals have been made as described in, for example, U.S. Pat. Nos. 3,898,084, 4,618,672, 4,396,696, 4,356,243, 4,743,523, 4,666,810, 4,436,800, 4,439,506, 4,447,513, 4,598,033, 4,568,623, 4,533,613, 4,619,881, and 4,702,982.
The organic electrophotographic photoreceptors are superior to the above-mentioned inorganic electrophotographic photoreceptors as the former can have somewhat improved mechanical characteristics and flexibility. However, the organic electrophotographic photoreceptors are generally poor in light sensitivity when used many times and therefore these are unsuitable for repeated use. Thus the above-mentioned organic substances cannot always sufficiently satisfy the requirements for electrophotographic photoreceptors.